Misc additions and improvemetns for Inkscape => PDF+LaTeX workflow

[wiesiwies]

I've made some changes which improve at least my work flow and I'd like to share.
The new LatexPassOn option allows LaTeX code to be retained in the SVG file. In combination with with the Inkscape => PDF+LaTeX export, plots with native LaTeX (and LaTeX font) rendering can be achieved.
A new GlobalScale option allows global scaling of the SVG output including all line widths (Same effect as if the resulting SVG is scaled i.e. in Inkscape).
An overlay bitmap image can now be placed onto the plot which can be used to generated complex hybrid bitmap/vector graphics.
Last but not least it is possible to rescale the patches, of e.g. surface plots. This is necessary with flat shading to prevent gaps between the individual patches.

I've included examples for each of the new functions and split them up into separate commits.